Make the chipotle sauce. Heat a large, dry nonstick frying pan, add the onion and cook for 5 minutes, turning frequently. Add the tomatoes and cook for a further 5 minutes, then add the garlic and continue to cook for 3 minutes, or until the ingredients are softened and charred. Transfer to a food processor or blender and whizz to a coarse paste. Leave to cool, then add the chipotle paste and season to taste. Set aside.
Place a layer of tortilla chips in a heatproof serving dish. Mix together the refried and black beans in a bowl, then spoon some of the beans over the chips and scatter with a layer of the chilli and cheese. Repeat the layers, finishing with a heavy layer of the cheese.
Bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 7 minutes, or until the cheese has melted.
Scatter over the avocado, tomato and coriander, drizzle with the chipotle sauce and soured cream and serve.